The ability of renal epithelia to reabsorb NaCl in a controlled manner is a crucial factor in maintaining salt and water balance in the body. In the distal nephron and related tissues, such as the amphibian urinary bladder, Na reabsorption is controlled to a large extent at the luminal membrane by the activity of channels or pores which permit the diffusion of Na from the urine into the epithelial cells. The channel activity is in turn influenced by (1) the concentration of Na in the urine, (2) hormonal status, especially that of aldosterone and antidiuretic hormone, and (3) intracellular factors such as ions and cyclic nucleotides. The goals of the project are to understand the physical/chemical mechanisms involved in the control of Na channel activity, using the amphibian urinary bladder as a model epithelium which can be studied in vitro. The extent to which both external and internal ions block the channel will be studied using current-voltage analysis and fluctuation analysis, to gain insight into the structure of the channel and its regulation. The role of changes in the internal concentrations of ions, including Ca and H, in the modulation of transport by hormones will be assessed using ion-selective microelectrodes to measure the ion activities. The possible involvement of endocytosis and exocytosis-like events in controlling the number of channels in the membrane will be examined using capacitance to estimate changes in membrane area. Finally, methodology for studying single Na channels in isolated membrane patches will be developed.